@Andrew - your question confused me a bit. I was thinking mostly from an admin perspective.
@esmi - I have run into a few scenarios where this would be useful.
1. A wordpress setup with the 'admin' user, with quite a few posts. I want to change the username for security reasons, as many suggest to do, but without having to create a new user, delete the original user, and assign his posts to the new user. Kind of a hassle.
2. A wordpress setup for a client where I misspelled one of the usernames, then that person created a bunch of posts before complaining that their username was misspelled.
I'm sure I've run into more situations, but cannot recall those now. I'll post later if I can think of them.
Tbh, I don't see a good reason NOT to allow this. However, I'm open to suggestions. Why do you guys think it's not a good feature to have by default?